1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a field sequential color mode liquid crystal display device including a temperature-compensating circuit and a driving method thereof that compensate temperature variations and enhance display qualities.
2. Discussion of the Related Art
Cathode ray tubes (CRTs) have been widely used in televisions and monitors for display images. However, CRTs have disadvantages in that they are heavy and large. In addition, CRTs require a high driving voltage especially when having a larger display area. Accordingly, flat panel display (FPD) devices, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, and organic electroluminescent display (ELD) devices have been the focus of recent researches because of their excellent characteristics of light weight and low power consumption.
In general, an LCD device is a non-self-emissive display device that displays images by controlling a transmittance of light emitted from a backlight unit through a liquid crystal panel. In particular, a cold cathode fluorescent lamp (CCFL) is widely used in the backlight unit for an LCD device. Such a backlight unit includes a lamp for emitting light, a lamp housing for surrounding the lamp, a light guiding plate for converting the light from the lamp into a plane light, a reflecting plate under the light guiding plate for upwardly reflecting downward and sideward light, a first diffusing sheet for diffusing the light from the light guiding plate, first and second prism sheets for adjusting a direction of light from the first diffusing sheet, and a second diffusing sheet for diffusing the light from the first and second prism sheets.
To form a small, thin and light-weighted backlight unit, a light emitting diode (LED) has been suggested to replaced the CCFL. An LCD device using a backlight unit having an LED may be driven using a field sequential color (FSC) driving method for obtaining a high display quality.
An FSC mode LCD device employs a light source including red-color, green-color and blue-color light sources, instead of a color filter layer having red, green and blue sub-color filters. In addition, in an FSC driving method, the red-color, green-color and blue-color light sources are sequentially turned on/off and an image of full color is displayed based on the persistence effect in human vision. Accordingly, one frame for displaying an image may be divided into three sub-frames that respectively correspond to red, green and blue color light emissions. Further, each light source is turned off during a time period of each sub-frame for writing a data and arranging liquid crystal molecules, and is turned on during the other time period of each sub-frame.
FIG. 1 is a schematic diagram showing a single time frame of a field sequential color (FSC) driving method for a liquid crystal display device according to the related art. In FIG. 1, one frame of about 16.7 ms is divided into three sub-frames of about 5.56 ms, R, G, and B, corresponding to red-color, green-color and blue-color light sources. Each of the sub-frames, R, G and B, is further divided into a first time period AP for inputting a data to a thin film transistor (TFT), a second time period WP for re-arranging liquid crystal molecules, and a third time period FP for emitting light using a light source including the red-color, green-color and blue-color light sources. Accordingly, each color-light source is turned on during the third time period FP of a respective sub-frame, but is turned off during the first and second time periods, AP and WP, of the respective sub-frame. As a result, the light source does not emit light during the entire duration of a frame.
In addition, an FSC mode LCD device may employ a light emitting diode (LED) in each of the color light sources, and in an FSC driving method, a data includes red, green and blue sub-data. Each sub-data is generated for one vertical sync time period, i.e., one frame, and the red, green and blue sub-data are sequentially supplied at an equal rate during the one vertical sync time period. As a result, the color-light sources do not simultaneously emit light, and the red-color, green-color and blue-color light sources are sequentially turned on. Since the red and green sub-data are supplied before the blue sub-data, red-color and green-color emissions need to be sustained for a longer period of time than blue color to obtain a white colored image. Thus, the light source is driven such that output intensities of the red-color and green-color light sources are higher than an output intensity of the blue-color light source, and a reduced response time of the liquid crystal molecules is required. For example, the first, second and third time periods AP, WP and FP of a sub-frame may be about 1.69 ms, about 1.5 ms and about 2.37 ms, respectively.
FIGS. 2A and 2B are images showing display qualities of an FSC mode liquid crystal display device according to the related art at different temperatures. FIG. 2A shows the display qualities of the FSC mode LCD device at a surrounding temperature of about 30° C., and FIG. 2B shows the display qualities of the FSC mode LCD device at a surrounding temperature of about −20° C. As shown in FIGS. 2A and 2B, when the surrounding temperature is about −20° C., the FSC mode LCD device produces an image having a lower contrast ratio and a lower color reproducibility in comparison to the image produced when the surrounding temperature is about 30° C. Such a decline in display qualities is caused by a deterioration of a switching element and an increase in response time of liquid crystal molecules when the LCD device is at a low temperature environment. Thus, the display qualities of an FSC mode LCD device considerably depends on temperature.
FIG. 3 is a collection of images showing display qualities of an FSC mode liquid crystal display device according to the related art at different temperatures. FIG. 3 represents blue, green, red, white and black images produced by the related-art FSC mode liquid crystal display device under a temperature range of about 20 to about −25° C. An upper portion and a lower portion of each image respectively correspond to a first gate line and a last gate line of the LCD device, respectively. That is, each image is produced by writing data from the upper portion to the lower portion of the FSC mode LCD device.
As shown in FIG. 3, the lower portion of the images displaying red and green colors begins to vary in color at about 5° C. and the lower portion of the images displaying blue color begins to vary in color at about 0° C. In addition, as the temperature further decreases, the whole portion of the FSC mode LCD device severely varies in color, because as the temperature decreases, a viscosity of liquid crystal molecules increases and a response speed of the liquid crystal molecules is reduced. Accordingly, the FSC mode LCD device of the related art does not display exact colors under a relatively low temperature. As a result, a contrast ratio and a color reproducibility of the FSC mode LCD device of the related art decrease as temperature varies, thereby deteriorating display qualities of the FSC mode LCD device.